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Saturday, June 4, 2011

Recording Tip 23: When using multiple mics, always check phase

This is a basic skill of engineering which I was taught early on. But in today’s world where many people begin recording at home, while it’s likely that some of you have heard of it, you may have no idea why you do it, or even what it means.

The reason for it is rather simple, but may sound complex. When you think of it, it makes total sense. Every sound has a positive pressure wave created. The drum, guitar, voice, whatever, makes a sound and the air molecules move from that sound source out into space. When a single microphone is struck by those moving air molecules, it’s converted to an electrical signal which goes to the mic preamp and into whatever interface you use for recording. Assuming all of your wiring is correct, it will then go thru that chain and come out your speakers with a positive push from your speakers. (The speaker moves toward you) So with one mic, all is well, generally speaking. (I’ll address this more in Recording tip #28 for when that might not be the case)

So again, assuming all is well with one mic, you’re fine. But if you use more than one mic you need to check for phase consistency. Why is that? The most obvious example for this is when you record drums. Here you typically have overhead mics, and mics on the toms, snare, high hat, and kick drum. When the drummer strikes the snare or toms, the pressure wave moves from the drum into the air, captured by the close mic as well as the overhead mics. Now hopefully all of the mics, the mic cables and preamps are all wired alike. Most times that will be the case. However, even if it is the case it’s possible to have the distant mics in such a position where they’re not capturing the pressure wave in the same cycle as the close mics. If that’s the case, when they are added together in the mix, you’ll find that certain frequencies will be cancelled to some degree. And if your cables or the mics themselves are wired wrong, this cancellation will happen as well.

The technical reason is this. Remember the sine wave graph from your math classes? From it’s beginning the waveform moves up into a positive form, then down to the negative part of the waveform and then back up. In a perfect scenario the sound pressure waves created when the drummer hits the snare or tom will be in alignment in both mics. But it’s possible that with the placement of the mics relative to one another, that might not happen. With one mic the waveform begins moving up to a positive form, and the other catches the waveform moving down. When that happens part of the frequencies will cancel when combined.

Everyone begins drum sounds differently. George Massenburg begins with the overheads and I do as well. I’ll ask the drummer play a basic groove; kick, snare, high hat. I’ll listen to the overheads and make sure they’re in phase. Now with experience you can tell if the overheads alone are in phase. But if you’re not sure, try flipping the phase of the overheads. When they are in phase you should get a solid center position of the snare drum between the speakers. ( Huge assumption here....I assume your speakers are in phase with each other...you need to have that right, but I’ll not get into that here.) Anyway, when I hear the overheads are good, I’ll mute them, and then go to the kick drum. Once I have a basic eq, I’ll then turn on the snare mic and listen with the overheads. But before I eq the snare I flip the phase button back and forth. When I hear a deeper sound of the snare that’s correct phase relationship. The same is true of the tom mics. I’ll have the drummer hit each drum and check phase before I begin to eq. To begin to eq before you check phase could make you backtrack because the change of phase will have a huge impact on how they sound, especially on the bottom end. That’s because the frequencies which are most affected by phase are the lower frequencies. Those frequencies will cancel more readily than others. What about checking phase with the kick drum mic? I will do that. But more times than not they don’t have those problems. However, if things aren’t sounding right, I’ll check that as well. As for the high hat, due to the high frequencies of the high hat, it’s in my experience never an issue. (Ok...yeah...never say never...but you get my drift)

Here’s a good tip in checking phase. When checking phase a good thing to do is to put the signals to mono. It can be easier, especially if you have tom mics panned hard left and right, to hear phase issues when in mono. Also, play with the balance between the snare, tom or whatever. If the close mic is too loud, you’ll not hear the potential phase issues.

By the way, an interesting thing to do after the drum have been recorded is to zoom in to sample level and compare waveforms of the close mics verses the overheads. You’ll clearly see that the waveform of the close mics are ahead in time as compared to the overhead mics. Some folks will move the overheads files so they’re in the same time as the close mics. I don’t advocate that. To me, that delay is a part of what creates the sound of a drum kit. We never could it in the analog days and arguably the best sounding records came from that era. So I don’t go down that rabbit hole. The same is true of some guys who do orchestra recordings. Some move the ambient mics in time to the close mics. I get that...but to me, that’s part of what makes things sound good. And as far as I’m aware, none of the best engineers in the business do that. And neither do I.

Now getting back to pop music...this phase issue is also true of recording guitars with multiple mics and especially a bass guitar when using a direct signal and an amp. The direct signal will get there first...pure physics. Most of the time you’ll not need to flip the phase of either. But you do check that. Sometimes it’s a clear 180 degree phase problem. Other times, it needs something in between. No matter which phase you pick, you hear some cancellation. If so, go move the mic on the amp. You need to find a place where the signals add in a positive way.

But if you get a mix where things don’t work with either in or out of phase, there are some solutions available. Littlelabs has a a plugin which is as far as I know, is only offered by Universal Audio. That plugin can move between the typical 180 degree phase shift. That can be a good solution if you have it. But if you don’t you can move the waveform of the mic. That’s the only situation where I will move a waveform and that’s to correct an error in the recording.

I hope this answers a bit of the mystery of phase issues. At times it can be a big deal. So be sure you check it out.

2 comments:

I think differentiating timing differences (the same source in two different, spaced mikes) and polarity (absolute push vs. pull with no timing difference) is important here. Calling both of these "phase" is confusing.

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Recording Engineer/Mixer/Producer living in Brentwood TN. 25+years in the biz. Lover of my family, the outdoors, photography, cooking, fine cigars and single malt scotch.
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